The present invention relates to organic light emitting diodes (OLEDs), and more particular to a modified OLED device wherein both edge emission and surface emission are used for backlighting. The modified OLED device of the present invention is a low weight, thin, and highly efficient backlight for use in a variety of displays, such as direct-view flat panel or virtual displays.
OLEDs are based on amorphous organic films deposited on any substrate, including non-crystalline substrates. OLED organic materials provide much cheaper solar cells than their LED counterparts since crystalline or epitaxially grown inorganic materials, required materials for LEDs, are costly. OLED organic films are typically very thin (on the order of about 50-100 nm, i.e., 500-1000 xc3x85) with respect to their LED counterparts, and combined with large Franck-Condon shift between absorption and emission, this slenderness makes them transparent to their own emission. Such transparency, in addition to the fact that the deposition of organic compounds in a vacuum may provide for large-area or custom shaped cells, makes novel display architectures and applications a propensity of OLED configurations.
In comparing an LED configuration to an OLED device it is important to note clear-cut differences in efficiency. OLEDs can achieve very high brightness of greater than 15,000 cd/m2 and have operational lifetimes greater than 10,000 hours when driven at video brightness (100 cd/m2) according to the Kirk-Othmer Encyclopedia of Chemical Technology, Fourth Edition, Supplemental volume, pages 488-495 (especially page 488). Thus, when merely based upon efficiency, there is a clear desire for more OLED technology.
A semiconductor LED structure matrix display is described in detail in U.S. Pat. No. 5,119,174 to Chen wherein it is taught that an LED crystal display maintaining bowl-punched die attachment zones on a printed circuit board (PCB) creates a reflector surface to concentrate the light emitting from the LED. However this LED is problematic in that the light, being matrixed, is incapable of being directed in a collimated manner and thus this device may not achieve the desired brightness. If such light were to be collimated, a grave concern arises as to the dispensation of the heat that would be produced.
The efficiency of a cold cathode fluorescent lamp has been reported by R. Y. Pai, xe2x80x9cEfficiency Limits for Fluorescent Lamps and Applications to LCD Backlighting,xe2x80x9d SID ""97 Digest, pp. 447-450, 1997, to be within the range from about 30 to about 80 lumens/watt (lm/W). On the other hand, the best efficiency of OLEDs reported in the literature are green: 16 lm/W, see, for example, T. Sano, et al., Extended Abstracts, 41st Spring Meeting, Japan Soc. of App. Phys., No. 3, p. 1073, 1994, blue: 6 lm/W, see, for example, C. Hosokawa, et al., xe2x80x9cOrganic Multicolor EL Display with Fine Pixels,xe2x80x9d SID ""97 Digest, pp. 1073-1076, 1997, and red: 2 lm/W, see, for example, P. E. Burrows, et al., xe2x80x9cAchieving Full-Color Organic Light-Emitting Devices for Lightweight, Flat Panel Displays,xe2x80x9d IEEE Transactions on Electron Devices, V.44, No. 8, pp. 1188-1203, 1997, respectively. In contrast to cold cathode fluorescent lamps, a white OLED combining these sources can only achieve an efficiency within the range of from about 15 to about 20 lm/W. A factor of 2 improvement in efficiency would enable OLEDs to compete with cold cathode fluorescent lamps in the future.
Furthermore, the on/off time of OLEDs is typically a microsecond or less, which is well-suited for time sequential red, blue, green illumination. This opens up the possibility of having color-filterless displays. The advent of such color-filterless displays having high efficiency would provide substantial savings for current color-filtered displays, a savings which could be passed along to the consumer.
Unlike semiconductor light emitting diodes, OLEDs exhibit substantially no resonant self-absorption because the peaks of the absorption band and emission band are separated by 0.4 eV, see, for example, P. E. Burrows, et al., xe2x80x9cRelationship Between Electroluminescence and Current Transport in Organic Heterojunction Light-Emitting Devices,xe2x80x9d J. App. Phys. 79(10), p. 7991, May, 15, 1997.
Prior art OLEDs of the type shown in FIGS. 1(a) and (b) are exclusively made to be surface-emitting diodes. Specifically, FIG. 1(a) shows an OLED structure where the anode is on top adjacent to the light emitting surface. Another type of structure as is shown in FIG. 1(b), where the cathode is on top, is described in detail in U.S. Pat. No. 5,739,545 to Guha, et al.
An advantage of these structures in FIGS. 1(a) and (b) is that they are easily fabricated in conventional silicon manufacturing facilities and may contain support circuitry in the silicon below the OLED structure. However, due to the fact that the OLED active layer (light generating layer) is fairly transparent relative to an LED active (or light generating) layer, some of the light that is confined within the light-generating layer is then transported (waveguided) to the edges of the chip and emitted. With an inorganic LED active layer this light would be mostly absorbed. However, prior art OLEDs in their current form meet the requirements for several low-resolution applications due to this loss of light via natural waveguidance but have yet to achieve higher resolution applicability.
In view of the inefficiencies of the prior art OLEDs, there is a continued need to develop new and improved OLED devices which increase illumination via an increase in the throughput of photons which would otherwise escape the OLED.
Accordingly, the present invention is directed to an organic light emitting diode (OLED) device comprising a substrate surface and sides for emission of photons therethrough and at least one adjacent light-folding means, wherein said adjacent light-folding means collects and redirects photons emitted from the sides of the OLED into coincident paths of light which are substantially parallel to the photons emitted by said substrate surface.
One object of the present invention is to provide an OLED device that has a higher optical efficacy as compared with prior art OLED devices.
Another object of the present invention is to provide an OLED device which has increased illumination as compared with prior art OLEDs.
A still further object of the present invention is to provide an OLED device which maintains a wave guide layer which guides light to the edges of the structure.
These and other objects and advantages are achieved by the present invention by utilizing an OLED device which combines the edge emission of photons with the normal surface emission of photons. By combining these two emission sources, the OLED device of the present invention exhibits high optical efficacy which is typically on the order of 2-3x above prior art OLEDs.
Specifically, the OLED device of the present invention comprises an OLED having a substrate surface and sides (or edges) for emission of photons therethrough and at least one adjacent light-folding means, wherein said adjacent light-folding means collects and redirects photons emitted from the sides of the OLED into coincident paths which are substantially parallel to the photons emitted through said substrate surface.
In another aspect of the present invention, a red, green and blue segmented OLED device for color sequential use is provided. In accordance with this aspect of the present invention, the color segmented OLED device comprises a plurality of color emitting OLEDs, each OLED having a substrate surface and sides for emission of photons therethrough, arranged sequentially in a predetermined order and at least one adjacent light-folding means, wherein said adjacent light-folding means collects and redirects photons emitted from the sides of the OLEDs into coincident paths which are substantially parallel to the photons emitted through said substrate surfaces.
Yet another object of the present invention is to provide an OLED device which can be used for providing time sequential pulses of various colors.
As a result of the teaching described herein, an OLED is able to be produced having the desired efficiency, brightness, and color stability that overcomes the disadvantages of the prior art.